Closed cycle cryogenic refrigerator



Aug. 1, 1967 CHELUS 3,333,433

CLOSED CYCLE] CRYOGENIC REFRIGERATOR Filed Jan. 26, 1966 2 Sheets-Sheet l REFRIGERATOR PRESSURE 70o USING BOOSTER CHAMBER 500 BOOSTER 7 Q CHAMBER I PRESSURE g 4oo- (n l 8 aoo- 1 REFRIGERATOR PRESSURE NO BOOSTER CHAMBER O I I I I l l l l l I I I I IO 20 3O 4O 5O 6O 7O 8O 90 I00 IIO I20 I30 TIME-MINUTES FROM START Fig. 4

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C INVEN'IOR. Fred F. CheHis Attorny Aug. 1, 1967 F. F. CHELLIS CLOSED CYCLE; CRYOGENIC REFRIGERATOR 2 Sheets-Sheet 2 Filed Jan. 26, 1966 I 4 V|\1 Jim i5| t M 5% m 2 w E J IQZ mm At orney United States Patent 3,333,433 CLOSED CYCLE CRYOGENIC REFRIGERATOR Fred F. Chellis, Manchester, Mass., assignor, by mesne assignments, to 500 Incorporated, Cambridge Mass., a

corporation of Delaware Filed Jan. 26, 1966, Ser. No. 523,159 6 Claims. (Cl. 62-6) ABSTRACT OF THE DISCLOSURE This invention relates to cryogenic refrigerators and more particularly to improving the efficiency of those cryogenic refrigerators which are constructed as completely closed units and which operate on a refrigerating fluid which is pressure cycled between chambers therein. A booster chamber containing pressurized refrigerating fluid is in fluid communication with the refrigerator through fluid-flow control means which are responsive to a decrease in pressure experienced by the refrigerator with cool-down and continual operation. By supplying additional pressurized fluid from the booster chamber the refrigerator is able to operate at higher pressures and with increased load-carrying ability.

There are many instances where it is desirable to have available relatively small, highly efficient cryogenic refrigerators, e.g., refrigerators delivering refrigeration from about to 80 K. In many cases, such refrigerators should be capable of being made an integral part of an apparatus such as a cryopump or an infrared detecting device; and should moreover be capable of delivering the desired amount of refrigeration while occupying a minimum volume. This in turn suggests the need for a highly eflicient refrigerator which exhibits a high load-carrying capability. Such refrigerators are normally of the closedcycle type having the means for compressing and expanding a fluid contained within a hermetically sealed vessel.

Those cryogenic refrigerators, which fall within the general class of refrigerators under consideration and to which this invention is applicable, are constructed to operate on one of two different cycles. For convenience,

these cycles are referred to hereinafter as the modified Taconis and the Philips-Stirling cycles.

Taconis in US. Patent 2,567,454 described a refrigerating system consisting of a gas-filled vessel fitted with three external heat exchangers, two regenerators and two displacers. The heat exchangers were designed to add a major portion of heat at 700 K., provide a minor amount of refrigeration at 100 K., and reject both heat fractions at 300 K. The working volumes were connected by passages opening through the heat exchangers and regenerators. The sequence of displacer movements was designed to be such that all volumes were nominally maintained at the same pressure at any instant and all of the fluid was transferred from the hot chamber to the cold chamber and thence to the intermediate-temperature chamber. However, there were a number of drawbacks to this cycle which are described in detail in Advances in Cryogenic Engineering, vol. 2, pages 188 et seq., published by Plenum Press, Inc., New York, 1960. A modified Taconis cycle disclosed therein differed from the original Taconis cycle in that it discharged the contents of the cold chamber at constant pressure thereby providing for the gas to leave that chamber at its lowest temperature. This required some change in displacer movement to cause the fluid to be discharged simultaneously from the top and bottom chambers into the midchamber. However, it was concluded that although the cycle was interesting, it was not feasible. Hogan in US. Patent 3,151,466 disclosed an improved apparatus operating on this modified Taconis cycle which did in fact make it feasible. This was done, in part, by constructing apparatus in which the various chamber volumes bore specific relationships to each other and the heat exchangers were formed in a unique arrangement. The Hogan apparatus was also constructed so that heat was supplied at room temperature, rejected at about 80 K. by boiling suflicient liquid nitrogen, and refrigeration developed down to about 10 K., thus in essence shifting the three temperature levels of Taconis. The

Hogan apparatus has proved to be a very efficient small refrigerator which can readily be integrated into equipment which requires localized refrigeration. Such equipment includes, but is not limited to, cryopumps, infrared detectors, and the like.

In the Philips-Stirling cycle (a typical apparatus of which is shown in US. Patent 2,657,553), a closed system is also employed. Compression of fluid is achieved through the mechanical actuation of a separate compressor piston in contrast to the use of thermal energy as in the modified Taconis cycle. Since in both of these cycles the fluid is alternately compressed and expanded within a closed vessel, the efliciency exhibited by these refrigerators is dependent at least to some extent upon the absolute values of the pressures through which the fluid is cycled. It will be appreciated that for any given fixed volume within the closed refrigerator vessel there is an upper practical pressure limit to which the fluid in the refrigerator may be charged. Normally, this will be in the range of about 750 p.s.i.g. As the refrigerator is operated and the cold volume reaches refrigerating temperature (cg, 15 K. with helium) and the temperature of the intermediale-temperature volume is stabilized at about K. the fluid pressure within the system decreases so that during the refrigerating cycle the fluid will be cycled between about 350 and 550 p.s.i.g. using the example of helium as the refrigerant operating in the Hogan apparatus. Thus, for a fixed volume, the pressure range, the efficiency and the loadcarrying ability of the cryogenic refrigerator of the class under consideration are fixed.

It would be highly desirable to be able to charge a closed refrigerator of this type and then to be able continuously to maintain the fluid pressure range at a higher level, once the chambers within the refrigerator had reached their operating temperatures. The invention described herein makes this type of operation possible by providing a fluid booster volume in controlled fluid communication with the volume of the refrigerator. The booster volume contains the refrigeration fluid under pressure and serves as a continuous source of additional pressurized fluid at a level of approximately the mean pressure existing in the refrigerator during operation. The result is to permit the refrigerator to operate at higher pressures when it is cooled down than is normally possible. This, in turn, results in increasing the refrigeration efficiency as well as its load-carrying ability. An additional advantage lies in the minimizing of the pressure difference across the seals in some modification.

It is therefore a primary object of this invention to provide a closed cryogenic refrigerator, operating by cycling a refrigerating fluid over a pressure range, which achieves higher operating pressures than normally possible. It is another object to provide a cryogenic refrigerator of the character described which has increased efliciency and load-carrying ability. Other objects of the invention will in part be obvious and will in part be apparent hereinafter.

The invention accordingly comprises the features of construction, combination of elements, and arrangement of paits which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which FIG. 1 is a simplified, somewhat diagrammatic drawing of a cryogenic refrigerator operating on the modified Taconis cycle;

FIG. 2 is a cross-section of the lower portion of the cryogenic refrigerator sketched in FIG. 1 embodying the booster volume of this invention;

FIG. 3 is a somewhat simplified cross-section of a cryogenic refrigerator operating on the Philips-Stirling cycle and embodying the booster volume of this invention; and

FIG. 4 is a plot of operating time vs. pressure for a modified Taconis cycle refrigerator with and without the booster volume.

For convenience of presentation, the invention will be described in detail as it is applied to a cryogenic refrigerator as described in US. Patent 3,151,466 issued to Hogan. Its applicability to a Philips-Stirling refrigerator will also be illustrated.

In FIG. 1 a cryogenic refrigerator constructed according to the teachings of U.S.P. 3,151,466 is illustrated in diagrammatic fashion inasmuch as the actual details of the refrigerator apparatus are not a part of the invention described herein. It will be seen that the refrigeration portion of the apparatus, generally designed by the numeral 10, is divided into three sections of decreasing diameter (from bottom to top) which are in axial alignment. Within the bottom section 11, which is sealed to a housing sleeve 14 (see FIG. 2), piston 12 defines with sleeve 14 a warm chamber 13. This piston is driven by rod 15. In the two upper sections 16 and 17 a piston formed of two connected sections 18 and 19 define two cold chambers 20 and 21 within the fluid-tight vessel. Associated and in thermal contact with the coldest chamber 21 is a heatstation 22 and refrigeration is delivered through the end plug 23. The movements of the stepped piston 18/19 and of piston 12 define the chamber 25 of intermediate temperature. Suitable coils 26 for circulating liquid nitrogen are wound about that portion of section 11 of the housing which corresponds to the maximum volume of chamber 25. Piston 18/19 is driven by a separate rod 28 which passes through the central section of piston 12. Fluid is transferred from chamber to chamber by a fluid flow path which is not shown in detail. It consists, however, of regenerators located internally of piston 12 and piston sections 18/ 19 such as regenerators 29 shown in FIG. 2. the flow paths are essentially those which are disclosed in U.S.P. 3,218,815 and incorporate the use of radial passages (such as 30 in FIG. 2) to provide fluid communication between the chambers and the regenerators. The refrigeration cycle need not be described further herein detail except to say that it is that which is carried out by the apparatus of U.S.P. 3,151,466.

FIG. 2 is a detailed cross-sectional representation of the lower portion of one embodiment of a cryogenic refrigerator constructed in accordance with the teachings of this invention. To simplify the drawing, a number of screws, bolts and other obvious design details have been omitted. FIG. 2 illustrates one possible mechanical driving mechanism and the use of a fluid-tight housing to enclose this mechanism and at the same time to serve as the fluid booster volume. It will be appreciated that other mechanical arrangements may be employed to effect the sequence of piston movement and that the one presented in FIG. 2 is illustrative only.

It will be seen in FIG. 2 that cylindrical sleeve section 14, which defines warm volume 13, is sealed in fluid-tight relationship with the bottom section 11 of the refrigerator housing through means of an O-ring seal which also serves to seal the refrigerator section and the auxiliary housing section to a mounting plate 36. The mounting plate 36 in turn makes it possible to mount the refrigerator into equipment so that the upper cold section can be inserted into the working portion of an apparatus, e.g., a cryopump, while the warm section can be exposed to the atmosphere.

The warm chamber 13 is terminated on the bottom by a heavy plate 38 which serves as the interface plate between the refrigerator section and the driving section. Sealing of the interface section 38 to the cylindrical section 14 is accomplished through suitable O-ring seals 39. It will be appreciated that upper and lower and like terms are used for convenience in describing the apparatus as it is oriented in the drawings. However, this arrangement is for illustrative purposes only since the refrigerator may be oriented in any desired position.

Piston rod 15 extends through the interface plate 38 within bushing 41 and a felt lubricating ring 42, and fluid scaling is accomplished through the use of an O-ring seal 43. In like manner, piston rod 28 extends through the interface plate 38 through bushing 45, a felt lubricating ring 46 and O-ring seal 47. Cylindrical sleeve section 14 is closed on its bottom end by an end plug 50 to which the interface section 38 is permanently bolted by means not shown. Within the end plug 50 is a fluid-tight chamber 52 defining a volume in which the mechanical driving means for the piston rods operate. These mechanical driving means may be of any suitable design and those shown in FIG. 2 are of the Scotch-yoke type. Associated with piston drive rod 15 is yoke 54 and a cam follower 55; and associated with piston drive rod 28 are yoke 56 and cam follower 57. Both of the cam followers are driven by the main motor shaft 72 through an auxiliary shaft 73 extending from shaft collar 74 and a double-throw crank 60 in accordance with known mechanical design techniques. The bottom shaft portion 62, connected with the yoke 55 and driving piston rod 15, and the bottom section 63 associated with piston rod 28 reciprocate within suitable recesses 64 and 65 through bushings 66 and 67. A port '70 is cut into the cylindrical sleeve section 14 to communicate with fluid chamber 52. Through this port passes the main motor shaft 72 which is used to drive the double-throw crank 60. The motor (not shown) is contained within a main motor housing 76 which, through collar 77 and an annular mounting plate 78, is afiixed in fluid-tight relationship to the cylindrical section 14. Sealing rings 80 and 81 are used to effect the necessary fluidtight seals. The main motor housing 76 has an end ring 85 and an associated sealing plug 86 and sealing ring 87.

The plug may be removed for pressurizing the motor housin g 76.

Within the motor housing 76 is a fluid-tight volume 90. which, it will be seen, is in fluid communication with volume 52 through the port 70. The combination of volumes 90 and 52 make up the booster chamber volume of this invention. In operation, these volumes are filled with the refrigerating fluid under pressure as will be explained below in connection with the description of the operation.

of the refrigerator.

Because the auxiliary booster volume is required to furnish refrigerating fluid at a controlled rate to the refrigerator volumes, it is necessary to provide fluid communication between the booster volume and the refrigerator volume as well as some means of controlling the flow of fluid. This fluid communication means may take several different forms. In FIG. 2, a fluid communication means is illustrated in the form of an external fluid conduit between volumes 52 and 13. Thus, there is provided a conduit 94, having a fluid flow control valve 98. This valve may be any suitable type of valve, preferably a needle valve. By proper operation of valve 98 the flow of fluid may be accurately controlled so as the fluid pressure drops in the refrigerator volume with the attainment of the operating temperature, additional fluid is introduced from the booster volume. If desired, pressure gages 102 and 103, with suitable valves 104 and 105, may be placed in the fluid conduit 94 to monitor the pressure within the system.

It is also possible to effect the necessary controlled fluid communication between the booster volume 90 plus 52 and volume 13 by means of a small orifice drilled in the interface plate 38. Another embodiment of the controlled fiuid flow communication between these two volumes lies in the use of seals (corresponding to either O-ring seal 43 or 47, or a combination of both) which are something less than fluid tight, thus in effect providing a controlled leak through these seals.

In FIG. 3 a typical Philips-Stirling refrigerator is schematically illustrated. As in the case of FIG. 1, much of the detail is not included since the refrigerator itself is not part of this invention. In the embodiment shown in FIG. 3 the booster volume is separate from that volume which contains the driving mechanism. In the refrigerator of FIG. 3 the refrigerating and driving mechanisms are con tained within a single fluid-tight housing 110 which s divided into a driving part 111 and a stepped refrigerator part made up of sections 112 and 113. Within this fluidtight housing, chamber 114 is isolated from chambers 115, 116, and 117 through appropriate fluid-tight sealing means. These latter chambers, during the operation of the refrigerator, are of course at succeeding lower temperature, refrigeration being delivered from the coldest chamher 117. A suitable fluid path is provided between chambers 115, 116, and 117 as illustrated diagrammatically. The fluid path incorporates regenerators 118 and 119 which are internal of the stepped piston 121. Fluid compression in chamber 115 is achieved through the operation of compressor piston 120 which is driven by piston rod 122. Stepped piston 121 is driven separately and in sequence through piston rod 123, both of the piston rods being connected to a suitable crank shaft 124 driven by an external motor 125. The booster volume 130, which contains the refrigeration fluid under pressure, is connected to the compressor chamber 115 through a suitable fluid conduit 131 which is controlled by valve 132.

FIG. 4 is a plot of the actual fluid pressures, from startup, which make up the pressure cycle in a refrigerator represented in the drawings in FIGS. 1 and 2. In FIG. 4 the dotted lines represent the pressure cycle of the apparatus when operated without the booster chamber; while the solid lines represent the pressure cycle of the refrigerator when operated with a booster chamber. The intermediate solid line represents the pressure which prevailed within the booster chamber.

It will be seen from FIG. 4 that when a closed cycle refrigerator constructed as shown in FIGS. 1 and 2 is pressurized with helium so that the initial fluid pressure Within the refrigerator prior to cool down is approximately 630 p.s.i., the final operating pressure is cycled between about 300 and 550 p.s.i. for a refrigerator which has no booster chamber associated with it. When, however, a booster chamber is used (in this case having a volume somewhat larger than the actual volume within the refrigerator) and it is initially pressurized with helium to the same pressure as the refrigerator, the final operating pressure in the refrigerator is cycled between about 400 and 650 p.s.i. In the case of the refrigerator used, the shifting of the pressure range upward by about 100 p.s.i. means that the load-carrying capacity of the refrigerator was substantially improved.

The actual fluid volume of the booster chamber relative to the actual fluid Volume of the refrigerator should be suflicient to contribute the required pressure stabilizing effect within the refrigerator. Moreover, it is desirable that fluid flow from the booster chamber to the refrigerator should be so controlled that the fluid pressure in the booster chamber should continue to approximate the mean pressure in the refrigerator as is the case in the performance curve shown in the solid lines in FIG. 4. In general, it will be preferable to provide booster chamber volumes which are at least equal to and even greater than the actual volume within the refrigerator.

It will be seen from the above description of the invention and from the performance characteristics shown in the plot of FIG. 4, that there is provided an improved closed cryogenic refrigerator. The improvement is to be seen in the ability of the refrigerator to maintain a higher pressure cycle with the same initial fluid pressure charge. This, In turn, imparts greater efliciency and an increase load-carrying capacity to the refrigerator.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in the ing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. A closed-cycle cryogenic refrigerator, comprising in combination (a) a fluid-tight housing;

(b) a plurality of chambers at different temperature levels within said housing, the volumes of which are defined by the movement of (c? fluid displacing means movable within said housmg and adapted to circulate a pressurized refrigerating fluid among said chambers;

(d) a fluid flow path incorporating heat exchange means connecting said chambers;

(e) driving means for moving said fluid displacing means through a predetermined cycle; and

(f) a fluid-tight booster chamber in fluid communication with said chambers through fluid-flow control means automatically responsive to a decrease in pressure within said refrigerator thereby to provide an additional quantity of said pressurized refrigerating fluid to said refrigerator during cool down and for continuing operating.

2. A cryogenic refrigerator according to claim 1 wherein said driving means are located within said booster chamber.

3. A cryogenic refrigerator according to claim 1 wherein said booster chamber is in fluid communication with said chambers through a valve-controlled fluid conduit.

4. A cryogenic refrigerator according to claim 1 wherein said booster chamber is in fluid communication with said chambers through an orifice.

5'. A cryogenic refrigerator according to claim 1 wherein said booster chamber is in fluid communication with said chambers through sealing means which are not fluid tight.

6. In a cryogenic refrigerator in which a pressurized refrigerating fluid is circulated among a plurality of variable-volume chambers within a fluid-tight housing by way of heat exchange means located in fluid flow paths providing fluid communication between said chambers, the improvement comprising a fluid-tight booster chamber pressurized with said refrigerating fluid and in fluid communication with said housing through fluid-flow control means automatically responsive to a decrease in pressure Within said refrigerator, thereby to provide an additional quantity of said pressurized refrigerating fluid to said refrigerator during cool down and for continuing operation.

References Cited UNITED STATES PATENTS 3,220,200 11/1965 Damsz 62---6 WILLIAM J. WYE, Primary Examiner.

above constructions without depart- 

1. A CLOSED-CYCLE CRYOGENIC REFRIGERATOR, COMPRISING IN COMBINATION (A) A FLUID-TIGHT HOUSING; (B) A PLURALITY OF CHAMBERS AT DIFFERENT TEMPERATURE LEVELS WITHIN SAID HOUSING, THE VOLUMES OF WHICH ARE DEFINED BY THE MOVEMENTS OF (C) FLUID DISPLACING MEANS MOVABLE WITHIN SAID HOUSING AND ADAPTED TO CIRCULATE A PRESSURIZED REFRIGERATING FLUID AMONG SAID CHAMBERS; (D) A FLUID FLOW PATH INCORPORATING HEAT EXCHANGE MEANS CONNECTING SAID CHAMBERS; (E) DRIVING MEANS FOR MOVING SAID FLUID DISPLACING MEANS THROUGH A PREDETERMINED CYCLE; AND (F) A FLUID-TIGHT BOOSTER CHAMBER IN FLUID COMMUNICATION WITH SAID CHAMBERS THROUGH FLUID-FLOW CONTROL MEANS AUTOMATICALLY RESPONSIVE TO A DECREASE IN PRESSURE WTHIN SAID REFRIGERATOR THEREBY TO PROVIDE AN ADDITIONAL QUANTITY OF SAID PRESSURIZED REFRIGERATING FLUID TO SAID REFRIGERATOR DURING COOL DOWN AND FOR CONTINUING OPERATING. 